Early signatures of ozone trend reversal over the Antarctic

We report on a detailed time series analysis of long total column ozone (TO) records based on multi-satellite observations of daily resolution. We concentrate on three geographic latitudes over and around the Antarctic area, specifically on three circles at 58.5°S, 59.5°S, and 79.5°S. Almost continuous observations are available at the two former latitudes; however, data are lacking during the polar winter periods at 79.5°S, because the measurement technique requires sunlight. The methodology is motivated by level-crossing statistics, where subsets of the records above or below particular threshold levels are evaluated. Long-term trend reversal at around the turn of the century is already detectable for low TO levels in the raw time series in the “ozone-hole” region (79.5°S). In order to overcome the apparent non-stationarities of the time series, we determined daily TO differences (ΔTO) belonging to the same geographic longitudes between the different latitudinal circles. The result is a stable, stationary behavior for small (absolute) ΔTO values in the period January–February–March without any significant detectable trends. The high absolute value ΔTO subsets (September–October–November) indicate a robust trend reversal in the middle of the 1990s. The observed trend reversal in the total column ozone time series is consistent with the temporal development of the stratospheric halogen loading. However, a close correspondence of ozone and halogen turnaround years is not expected because of the statistical uncertainties in the determination of the ozone turnaround, and the many factors contributing to ozone depletion processes